BOSTON -- Federal agents zeroed in Tuesday on how the Boston Marathon bombing was carried out -- with kitchen pressure cookers packed with explosives, nails and other lethal shrapnel -- but said they still didn't know who did it and why.
The FBI and other law enforcement agencies repeatedly appealed to the public to come forward with photos, videos or anything suspicious they might have seen or heard.
"The range of suspects and motives remains wide open," Richard DesLauriers, FBI agent in charge in Boston, said at a news conference. He vowed to "go to the ends of the Earth to identify the subject or subjects who are responsible for this despicable crime."
President Barack Obama branded the attack an act of terrorism but said officials don't know "whether it was planned and executed by a terrorist organization, foreign or domestic, or was the act of a malevolent individual."
He added: "The American people refuse to be terrorized."
The bombs exploded 10 or more seconds apart, tearing off victims' limbs and spattering streets with blood, instantly turning the festive race into a hellish scene of confusion, horror and heroics.
The blasts killed 8-year-old Martin Richard of Boston, 29-year-old Krystle Campbell of Medford, Mass., and Lu Lingzi, a graduate student at Boston University.
Richard's mother, Denise, and 6-year-old sister, Jane, were badly injured. His father, Bill, and brother also were watching the race but were not hurt.
"My dear son Martin has died from injuries sustained in the attack on Boston. My wife and daughter are both recovering from serious injuries. We thank our family and friends, those we know and those we have never met, for their thoughts and prayers," Bill Richard said in a statement released Tuesday. "I ask that you continue to pray for my family as we remember Martin. We also ask for your patience and for privacy as we work to simultaneously grieve and recover. Thank you."
A candle burned on the stoop of the family's single-family home in the city's Dorchester section Tuesday, and the word "Peace" was written in chalk on the front walk.
Neighbor Betty Delorey said Martin loved to climb neighborhood trees and hop the fence outside his home.
Campbell's father, William, said Tuesday that Krystle had gone with her best friend to take a picture of the friend's boyfriend crossing the finish line on Monday afternoon.
His daughter, who worked at a restaurant in nearby Arlington, was a "very caring, very loving person, and was daddy's little girl," William Campbell said. He said the loss has devastated the family.
A state-run Chinese newspaper identified Lu, originally from China's northeastern city of Shenyang. An editor at the Shenyang Evening News reported says that Lu's father confirmed his
daughter's death when reporters visited the family home.
Scores of victims of the bombing remained in hospitals, many with grievous injuries, a day after the twin explosions near the marathon's finish line wounded more than 170 and reawakened fears of terrorism. A 9-year-old girl and 10-year-old boy were among 17 victims listed in critical condition.
Doctors who treated the wounded corroborated reports that the bombs were packed with shrapnel intended to cause mayhem.
"We've removed BBs and we've removed nails from kids. One of the sickest things for me was just to see nails sticking out of a little girl's body," said Dr. David Mooney, director of the trauma center at Boston Children's Hospital.
At Massachusetts General Hospital, all four amputations performed there were above the knee, with no hope of saving more of the legs, said Dr. George Velmahos, chief of trauma surgery.
"It wasn't a hard decision to make," he said. "We just completed the ugly job that the bomb did."
Heightening jitters in Washington, where security already had been tightened after the bombing, a letter addressed to a senator and poisoned with ricin or a similarly toxic substance was intercepted at a mail facility outside the capital, lawmakers said.
There was no immediate indication the episode was related to the Boston attack. Senate Majority Leader Harry Reid said the letter was sent to Republican Sen. Roger Wicker of Mississippi.
Officials found that the bombs in Boston consisted of explosives put in ordinary, 1.6-gallon pressure cookers, one with shards of metal and ball bearings, the other with nails, according to a person close to the investigation who spoke to The Associated Press on condition of anonymity because the probe was still going on.
Both bombs were stuffed into black duffel bags and left on the ground, the person said.
DesLauriers confirmed that investigators had found pieces of black nylon from a bag or backpack and fragments of BBs and nails, possibly contained in a pressure cooker. He said the items were sent to the FBI laboratory at Quantico, Va., for analysis.
Investigators said they have not yet determined what was used to set off the Boston explosives.
"It was a war zone. Truly," runner Demi Clark, who was steps shy of the finish line when the first explosion occurred, told ESPN's Marty Smith. "It was triage immediately. I would not even know what it's like to be in Afghanistan, but this was a war zone."
FBI agents searched an apartment in the Boston suburb of Revere overnight, and investigators were seen leaving with brown paper bags, plastic trash bags and a duffel bag. But it was unclear whether the tenant had anything to do with the attack.
A law enforcement official who spoke on condition of anonymity because he was not authorized to release details of the investigation told The Associated Press that the man had been tackled by a bystander, then police, as he ran from the scene of the explosions.
But the official said it is possible the man was simply running away to protect himself from the blast, as many others did.
According to The Boston Globe, the man's roommate, Mohammed Bodawood, said Tuesday he doubts his friend played a role in the attack.
"I don't think he could do that," Bodawood told the newspaper.
Bodawood described his roommate as a 20-year-old devout Muslim from Saudi Arabia and a soccer fan.
A spokesman for the Saudi embassy in Washington also said U.S. officials had told the embassy Monday night that the man was not a suspect and was cooperating with the investigation.
Pressure-cooker explosives have been used in international terrorism, and have been recommended for lone-wolf operatives by Al-Qaida's branch in Yemen.
But information on how to make the bombs is readily found online, and U.S. officials said Americans should not rush to judgment in linking the attack to overseas terrorists.
DesLauriers said that there had been no claim of responsibility for the attack.
He urged people to come forward with anything suspicious, such as hearing someone express an interest in explosives or a desire to attack the marathon, seeing someone carrying a dark heavy bag at the race, or hearing mysterious explosions recently.
"Someone knows who did this," the FBI agent said.
Obama plans to visit Boston on Thursday to attend an interfaith service in honor of the victims. He has traveled four times to cities reeling from mass violence, most recently in December after the schoolhouse shooting in Newtown, Conn.
In the wake of the attack, security was stepped up around the White House and across the country. Police massed at federal buildings and transit centers in the nation's capital, critical response teams deployed in New York City, and security officers with bomb-sniffing dogs spread through Chicago's Union Station.
Homeland Security Secretary Janet Napolitano urged Americans "to be vigilant and to listen to directions from state and local officials." But she said there was no evidence the bombings were part of a wider plot.
Pressure-cooker explosives have been used in Afghanistan, India, Nepal and Pakistan, according to a July 2010 intelligence report by the FBI and the Homeland Security Department. One of the three devices used in the May 2010 Times Square attempted bombing was a pressure cooker, the report said.
"Placed carefully, such devices provide little or no indication of an impending attack," the report said.
The Pakistani Taliban, which claimed responsibility for the 2010 attempt in Times Square, has denied any part in the Boston Marathon attack.
Investigators in the Boston bombing are also combing surveillance tapes from businesses around the finish line and asking travelers at Logan Airport to share any photos or video that might help.
"This is probably one of the most photographed areas in the country yesterday," said Boston Police Commissioner Edward Davis. He said two security sweeps of the marathon route had been conducted before the bombing.
Boston police and firefighter unions announced a $50,000 reward for information leading to arrests.
Gov. Deval Patrick said that contrary to earlier reports, no unexploded bombs were found. He said the only explosives were the ones that went off.
At a news conference Tuesday, police and federal agents repeatedly appealed for any video, audio and photos taken by marathon spectators, even images that people might not think are significant.
The race winds up near Copley Square, not far from the landmark Prudential Center and the Boston Public Library. It is held on Patriots Day, which commemorates the first battles of the American Revolution, at Concord and Lexington in 1775.
Organizers of sporting events in the coming weeks are devoting extra attention to their security measures.
• The NFL will review security measures in place for its draft to see if any additional reinforcements are necessary, the league said. The draft is April 27-29 in New York.
• Fans attending NASCAR events at Kansas Speedway this weekend should expect slightly longer waits for security checks as a part of heightened security.
"We've had several meetings with our local contact with the Kansas police department which interphases with Homeland Security and the FBI," track president Pat Warren told ESPN. "We don't discuss publicly the things we do because we don't want somebody who might do something bad to know what our plans, policies and procedures are."
• Brazil's foreign minister Antonio Patriota said "all necessary measures" are being taken to ensure security at next year's soccer World Cup and the 2016 Olympics, adding he is confident the country's security measures "will guarantee the security of the events."
Rio will also host two major events later this year, the Confederations Cup soccer tournament and the World Youth Day, a Roman Catholic pilgrimage that's expected to be attended by Pope Francis and as many as 2.5 million visitors.
About 23,000 runners participated in this year's Boston Marathon. Nearly two-thirds of them had crossed the finish line by the time the bombs exploded, but thousands more were still completing the course, and the area around the finish line was crowded with athletes and friends and relatives cheering them on.
The Boston Athletic Association, the group that organizes the marathon, released a statement Tuesday extending sympathy to the victims of the explosions. The B.A.A. also stated that it plans to run the Boston Marathon again next year.
"Boston is strong," Thomas Grilk, the B.A.A. executive director, said in the statement. "Boston is resilient. Boston is our home. And Boston has made us enormously proud in the past 24 hours. The Boston Marathon is a deeply held tradition -- an integral part of the fabric and history of our community. We are committed to continuing that tradition with the running of the 118th Boston Marathon in 2014."
Information from The Associated Press and ESPN.com senior writer David Newton was used in this report.